1. Field of the Invention
This invention relates to rotary actuators of the type in which axial movement of a piston produces rotary motion of an output member.
2. Description of the Prior Art
Rotary helical actuators have been employed in the past and offer the advantages of high torque output capabilities from a simple linear piston and cylinder drive arrangement. One typical helical actuator is shown in U.S. Pat. No. 3,255,806. One of the shortcomings of these rotary helical acutators, however, is that the rotary output is determined to a large extent by the length of travel of the linearly moved internal piston. That is, the greater the length of movement of the piston, generally the greater the amount of rotational output for a given helical angle.
Rotary actuators are frequently applied to high torque load applications wherein the shaft of the actuator is held stationary and the cylinder or housing rotated or wherein the outer cylinder or housing is held stationary and the shaft is rotated.
Applications of helical actuators have been limited because the overall length of the actuator was too great for the space requirements and for the torque desired. A specific example is the hinge-pin for the articulated boom of a conventional backhoe or the like. This type of boom mounting has a short vertical pivot pin for allowing the boom to swing laterally. A linear hydraulic actuator is the coupled to the side of the boom for providing the lateral swinging. In some instances, this vertical pivot pin has been itself a rotary actuator and could be a helical rotary actuator. The length requirements, the large radial loads and the torque requirements for a pivot pin for such a boom have generally precluded, however, the use of a helical actuator in that location, since to provide the necessary bearings for swinging the boom when at full load, the actuator with its bearings between shaft and the cylinder have made the actuator too long for that application. Thus, it is this combined requirement of short length, large external radial or moment load capability and high torque capacity that has limited the useful applications for these rotary helical actuators.